188 MUSCLE AND NERVE 



is wavy or unbroken, it is known as an incomplete or complete 

 tetanus. 



Tetanus is the result of three factors : 



1. Increase of irritability. 



2. Summation of contractions. 



3. Support offered by contracture. 



FIG. 11 



Serial contractions of gastrocnemius of frog, showing double contrac- 

 ture, introductory contraction, staircase, and fatigue. Two stimuli per 

 second, maximal induced current. Muscle weighted with 10 grams and 

 used three days after pithing frog. 



Final portion of curve A, showing gradual relaxation due to fatigue; 

 x, cessation of stimulation. 



All normal physiological contractions must be regarded as 

 short tetani, and the normal impulse as a discontinuous form 

 of excitation. As evidence of this may be cited the fact that 

 muscles give out a sound when contracting, implying that 

 their finest particles are in a state of vibration. Wollasten 

 determined the rate to be from 36 to 40 per second. By means 

 of vibrating reeds Helmholtz reduced the rate 18 to 20 per 

 second, which gives a tone imperceptible to the ear. Lately 

 the rate has been studied by a new and much more reliable 

 method. Each separate stimulus to a muscle causes a distinct 

 electrical variation. By means of a string galvanometer these 

 electrical changes may be registered and their rate determined. 



